Segment Information (Details) $ in Thousands	3 Months Ended				6 Months Ended
	Mar. 31, 2020 USD ($) state	Dec. 31, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Mar. 31, 2020 USD ($) state	Mar. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)
Segment Reporting Information [Line Items]
Number of states with service areas | state	8				8
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	$ 977,665		$ 1,094,645		$ 1,853,228	$ 1,972,427
Purchased gas cost	317,883		471,676		614,751	813,841
Operation and maintenance expense	147,824		149,427		300,069	288,027
Depreciation and amortization expense	105,916		96,772		210,978	192,837
Taxes, other than income	74,604		79,093		143,211	143,581
Operating income	331,438		297,677		584,219	534,141
Other non-operating income (expense)	(2,989)		4,232		1,898	(3,491)
Interest charges	22,171		26,949		49,400	54,798
Income before income taxes	306,278		274,960		536,717	475,852
Income tax (benefit) expense	66,632		60,072		118,398	103,318
Net income	239,646	$ 178,673	214,888	$ 157,646	418,319	372,534
Capital expenditures	465,551		361,182		994,737	777,586
Segment Reporting Information, Balance Sheet [Abstract]
Property, plant and equipment, net	12,548,240				12,548,240		$ 11,787,669
Total assets	14,716,398				14,716,398		13,367,619
Distribution
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	932,296		1,057,192		1,760,136	1,895,373
Pipeline and Storage
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	45,369		37,453		93,092	77,054
Operating Segments | Distribution
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	933,005		1,057,889		1,761,509	1,896,724
Purchased gas cost	418,935		570,348		816,493	1,008,080
Operation and maintenance expense	115,851		117,621		230,203	223,388
Depreciation and amortization expense	76,265		69,904		152,339	139,613
Taxes, other than income	68,413		71,053		128,656	127,243
Operating income	253,541		228,963		433,818	398,400
Other non-operating income (expense)	(5,191)		5,263		(3,237)	(1,214)
Interest charges	10,797		15,896		27,159	34,106
Income before income taxes	237,553		218,330		403,422	363,080
Income tax (benefit) expense	50,489		46,137		86,601	76,502
Net income	187,064		172,193		316,821	286,578
Capital expenditures	373,313		293,270		777,560	595,815
Segment Reporting Information, Balance Sheet [Abstract]
Property, plant and equipment, net	9,364,424				9,364,424		8,737,590
Total assets	13,946,651				13,946,651		12,579,741
Operating Segments | Pipeline and Storage
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	146,237		135,650		294,413	270,120
Purchased gas cost	202		(90)		301	(448)
Operation and maintenance expense	32,296		32,118		70,517	65,265
Depreciation and amortization expense	29,651		26,868		58,639	53,224
Taxes, other than income	6,191		8,040		14,555	16,338
Operating income	77,897		68,714		150,401	135,741
Other non-operating income (expense)	2,202		(1,031)		5,135	(2,277)
Interest charges	11,374		11,053		22,241	20,692
Income before income taxes	68,725		56,630		133,295	112,772
Income tax (benefit) expense	16,143		13,935		31,797	26,816
Net income	52,582		42,695		101,498	85,956
Capital expenditures	92,238		67,912		217,177	181,771
Segment Reporting Information, Balance Sheet [Abstract]
Property, plant and equipment, net	3,183,816				3,183,816		3,050,079
Total assets	3,403,106				3,403,106		3,279,323
Intersegment eliminations
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	(101,577)		(98,894)		(202,694)	(194,417)
Purchased gas cost	(101,254)		(98,582)		(202,043)	(193,791)
Operation and maintenance expense	(323)		(312)		(651)	(626)
Depreciation and amortization expense	0		0		0	0
Taxes, other than income	0		0		0	0
Operating income	0		0		0	0
Other non-operating income (expense)	0		0		0	0
Interest charges	0		0		0	0
Income before income taxes	0		0		0	0
Income tax (benefit) expense	0		0		0	0
Net income	0		0		0	0
Capital expenditures	0		0		0	0
Segment Reporting Information, Balance Sheet [Abstract]
Property, plant and equipment, net	0				0		0
Total assets	(2,633,359)				(2,633,359)		$ (2,491,445)
Intersegment eliminations | Distribution
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	709		697		1,373	1,351
Intersegment eliminations | Pipeline and Storage
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	$ 100,868		$ 98,197		$ 201,321	$ 193,066